In many parts of the subhumid and semiarid tropics, crop
yields are declining on response to inputs such as fertilizers, and droughts and
shortages of irrigation water are increasingly evident.

Sub-Saharan Africa and Asia pose two different challenges in
raising food production to meet their food needs:

Much of the
agriculture in sub-Saharan Africa and Asia is not irrigated but rainfed, with
the associated uncertainty as to the onset, reliability and amounts of rainfall
each year. Limitations to the expansion of the existing cultivated area are
increasingly those of uncertain rainfall and of the topographic and chemical
hazards associated with taking marginal lands into cultivation. In sub-Saharan
Africa a growth rate in food production of about 4 percent per year is needed to
keep up with population growth to 2030, whereas over past years only about 2
percent rate per year has been achieved (FAO, 1996b).

The main crop in much of Asia
is rice, both irrigated and rainfed in wet valley lands and also grown on
rainfed uplands. Three major problems limit the expansion of irrigated rice
production into the future: (i) increasing competition for irrigation water from
non-agricultural uses; (ii) even with the development of high-yielding rice
varieties and hybrids yields are rising more slowly than previously and appear
to be approaching a plateau of productive potential; (iii) only about 5 percent
of the original total of potentially irrigable land might still remain by 2030
for expansion of irrigated rice (except in India) (FAO, 2000a).

The problem of lower annual increases in yield per hectare is
a not confined to irrigated rice. Wheat and maize are also apparently reaching
similar plateaus. For the three major staple crops (paddy rice, wheat and maize)
the average yield increases between 1963 and 1983 were respectively, 2.1, 3.6
and 2.9 percent per year; but in the 10 years 1983-1993 the rates of increase
had fallen respectively to 1.5, 2.1 and 2.5 percent per year (FAO,
1996b).

Some 40 percent of all food is produced under irrigation, from
about 18 percent of the world's area of arable land plus permanent crops, with
60 percent produced under rainfed agriculture. As populations have risen this
arable land and permanent crops area per head has been falling, except in the
case of Europe (Table 1).

TABLE 3Decline in response of local maize to
fertilizers in Malawi (Malawi Government, 1957 - 1985)

District

Mean response rate1957 - 1962(kg
maize/kg N)

Mean response rate1982 - 1985(kg
maize/kg N + P2O5)

Lilongwe

23

13

Kasungu

24

18

Salima

25

17

Mzuzu

32

18

In the State of Paraná in southern Brazil, from the
time of clearing the land from native forest decades ago, yields of crops under
conventional tillage fell between 5 to 15 percent in 10 years. This was
accompanied by severe losses of soil, associated organic matter and applied
nutrients, resulting in downstream flooding, sedimentation and other damage
(Plates 1 and 2).

Records from Lesotho show that mean yields of major crops
(generally without added fertilizers) declined between 1978 and 1986, related to
a combination of adverse weather conditions, decline in soil conditions, and
recurrent erosion and runoff. A three-year running mean of yields has been used,
in order to smooth the effects of between-year weather variations (Table
4).

At the same time, it has been widely observed that ongoing
land degradation across topographic catchments has resulted in increasingly
irregular streamflow, with more floods of muddy water in the rainy season and
declining volume and duration of streamflow during the dry season (Plates 3, 4
and 5).

Human-induced agricultural land degradation is widespread in
irrigated and rainfed land and in both tropical and temperate zones. Land
degradation represents a challenge to the sustainability of farming systems in
all regions, even those of low population densities (after FAO, 2001a).
In rainfed lands, compaction, erosion and runoff are significant problems. On
irrigated lands, problems are often those of poor drainage control, salinization
and compaction leading to nutrient deficiencies (Figure 1).

Such problems are not confined to tropical areas.
Clear-felling of trees (Plate 6), grazing on very steep slopes (Plate 7) and the
compacting effects of farm machinery (Plate 8) result in excess water runoff and
erosion in temperate zones. Plate 9 shows a soil that has been compacted at
about 8-10 cm depth by repeated disking, which has the effect of reducing its
effective depth. Under native vegetation, this soil is deep and water
absorptive. The difference in the growth of the soybean plants between the top
left and upper right of the photo can be related both to the effects of erosion
and to induced soil moisture shortage in the root zone.

PLATE 1. Clear water, from
stable absorptive land. Cerrado, Brazil

[T.F. Shaxson]

Estimates of damage caused by compaction and erosion in the
Eurasian region suggest that about 327 million hectares of land in Eurasia have
been severely affected by wind and water erosion. Approximately 170 million ha
of land have been affected by soil compaction. Conservative estimates calculate
a production loss of 15 million tons of grain, two million tons of sugar beet
and 500 000 tons of maize. Others calculate a 16-27 percent decrease in
production as a result of compaction, with a loss of 50 million tons of grain
production alone (Karabayev et al., 2000).

PLATE 2. This floodwater is
turbid with eroded soil: it did not enter the ground first to emerge clean into
the river. Caledon River, Lesotho

[T.F. Shaxson]

PLATE 3. The Namadzi stream,
arising in a poorly managed cultivated hilly catchment, is empty of water in the
dry season. Namadzi, Malawi

[T.F. Shaxson]

PLATE 4. From the same
viewpoint upstream of the road-bridge, in the rainy season just after a storm
the Namadzi stream is a raging, soil-filled torrent. Namadzi,
Malawi

[T.F. Shaxson]

PLATE 5. These mature trees
must have grown up along and above the riverbank. But destruction of soil
porosity and permeability by bad husbandry in the catchment has resulted in
heightened flood peaks, which have eroded away the riverbanks and left the trees
marooned in the middle of the river bed. Mikolongwe, Malawi

[T.F. Shaxson]

PLATE 6. Clear-felling of
planted forest and destruction of ground cover on steep slopes bares the soil
and encourages runoff and erosion. Palmerston North, New Zealand

[T.F. Shaxson]

PLATE 7. Reduction of ground
cover due to severe grazing by sheep causes landslips, with loss of plants,
water and soil. Palmerston North, New Zealand

[T.F. Shaxson]

PLATE 8. Use of heavy farm
machinery can compact soil and encourage runoff even where rainfall is never
very intense. Abbotsbury, England

[T.F. Shaxson]

A study of the effects of soil compaction on wheat production
in New Zealand showed that as the soil becomes increasingly degraded, costs rise
as yields fall, squeezing margins of profit per hectare (Shepherd, 1992). This
indicates a wider problem that, as yields begin to decline, farmers may apply
more fertilizer, masking the underlying decline into unsustainable and
uneconomic production. A survey of small resource-poor farmers in central
Paraguay showed that as erosion and runoff continued, yields of cotton, tobacco,
maize and other crops declined. As a result net farm incomes fell and farmers
could no longer afford to buy equipment or inputs that might help to reverse the
downward trend. This has led to farms being abandoned and desperate families
migrating to the cities in search of income that farming could no longer provide
(Sorrenson et al., 1998).

PLATE 9. Effects of compaction
on root habitat of soybean

[T.F. Shaxson]

Potential sources of growth in overall output are: (1)
expansion of arable land area (2) increases in cropping intensities to give
greater harvested area; (3) growth in yield per hectare (FAO, 2000a).
Considering the present problems with production noted above, these expectations
are optimistic if areas of already-damaged soils continue to be managed in the
same way as in the past. Expansion of arable land will be limited because almost
all lands of good and moderate quality have been settled. Expansion of the
cultivated area will be onto land with increasing difficulties and hazards,
which will reflect negatively on the yields and economics in crop production of
both rainfed and irrigated crops. Increases in cropping intensity with shorter
(or even no) regular recuperative periods during which damaged soils can recover
their soil fertility will result in continued and worsening land degradation.
Rates of yield increase are tending to fall and upper limits of potential yield,
at least of key grain crops, are apparently being approached where high inputs
are used on the best soils.

A way forward

Intensification is the only option to increase usable biomass
and available water per unit area of land. The challenge is to achieve
intensification without causing more damage to soils and to the quantity and
reliability of water supplies. Unfortunately, past attempts to intensify
production using conventional methods have often resulted in damage to the
soil.

The sustainability of agriculture depends not only on the soil
continuing to be a fit place for crops, pastures and trees, but also on young
people being enthused by farming to provide a continuity from one generation to
the next, developing and carrying forward up-to-date knowledge and relevant
skills in the husbandry of plants, animals and land.

How can farming return to being a way of life which is
satisfying to many and that encourages them to remain in the rural areas? How
can sufficient water be ensured, both for plant growth and for the regular flow
of rivers, when much recent experience shows increasingly severe effects of
drought on crop plants and a decline in the regularity and volume of river flow?
How to achieve not only greater total output, but also better quality and
improved food security over the year? How to produce a greater variety of foods
to improve nutrition and health and reduce poverty by generating income?
Conventional approaches seem to be inadequate for the task, despite the efforts
of many to date.

Key parts of any strategy to address these issues
include:

Recognizing the
soil as a key and living component of the environment. To date it has
received far less attention in comparison with the above-ground components,
which are more readily perceived.

Encouraging the inherent
capacities of life itself. Particularly the ability of bacteria, fungi, soil
fauna and plants to continually colonize and modify habitats.

Prolonging the usefulness
of rainwater and organic matter. By recycling through different biotic
processes as many times as possible.

In 1971 D.A. Poole wrote:

"We must begin to regard our individual disciplines as part
of a whole - an ecological whole - as one of the several moving parts that,
depending on how applied, either catalyses or obstructs the working of the
whole. We must recognize and promote the ecology of our individual disciplines,
none of which can afford to act alone. The public cares less about the technical
aspects of soil conservation, forestry, wildlife or any other discipline than it
does about their environmental effects. People will support - with money and
voices - those professionals whose programs truly assure them of environmental
improvement. And they will resist - as they are doing so strongly today - those
programs based more on textbook philosophy than on environmental
acceptability."